Liquid-crystalline medium

ABSTRACT

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, characterized in that it comprises one or more compounds of the formula I  
                 
 
     in which R 1 , L 1  and L 2  are as defined in claim 1.

[0001] The present invention relates to a liquid-crystalline medium, tothe use thereof for electro-optical purposes, and to displays containingthis medium.

[0002] Liquid-crystals are used principally as dielectrics in displaydevices, since the optical properties of such substances can be modifiedby an applied voltage. Electro-optical devices based on liquid crystalsare extremely well known to the person skilled in the art and can bebased on various effects. Examples of such devices are cells havingdynamic scattering, DAP (deformation of aligned phases) cells,guest/host cells, TN cells having a twisted nematic structure, STN(supertwisted nematic) cells, SBE (superbirefringence effect) cells andOMI (optical mode interference) cells. The commonest display devices arebased on the Schadt-Helfrich effect and have a twisted nematicstructure.

[0003] The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

[0004] They should furthermore have a suitable mesophase, for example anematic or cholesteric mesophase for the above-mentioned cells, at theusual operating temperatures, i.e. in the broadest possible range aboveand below room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

[0005] For example, for matrix liquid-crystal displays with integratednon-linear elements for switching individual pixels (MLC displays),media having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and lower vapor pressure are desired.

[0006] Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, active elements (i.e. transistors). The term“active matrix” is then used, where a distinction can be made betweentwo types:

[0007] 1. MOS (metal oxide semiconductor) or other diodes on a siliconwafer as substrate.

[0008] 2. Thin-film transistors (TFTs) on a glass plate as substrate.

[0009] The use of single-crystal silicon as substrate material restrictsthe display size, since even modular assembly of various part-displaysresults in problems at the joins.

[0010] In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. Intensive work is being carried out world-wide on thelatter technology.

[0011] The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully color-capabledisplays, in which a mosaic of red, green and blue filters is arrangedin such a way that a filter element is opposite each switchable pixel.

[0012] The TFT displays usually operate as TN cells with crossedpolarizers in transmission and are illuminated from the back.

[0013] The term MLC displays here covers any matrix display withintegrated non-linear elements, i.e., besides the active matrix, alsodisplays with passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

[0014] MLC displays of this type are particularly suitable for TVapplications (for example pocket TVs) or for high-information displaysfor computer applications (laptops) and in automobile or aircraftconstruction. Besides problems regarding the angle dependence of thecontrast and the response times, difficulties also arise in MLC displaysdue to insufficiently high specific resistance of the liquid-crystalmixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E.,SORIUMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay84, September 1984: A 210-288 Matrix LCD Controlled by Double StageDiode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84,September 1984: Design of Thin Film Transistors for Matrix Addressing ofTelevision Liquid Crystal Displays, p. 145 ff, Paris]. With decreasingresistance, the contrast of an MLC display deteriorates, and the problemof after-image elimination may occur. Since the specific resistance ofthe liquid-crystal mixture generally drops over the life of an MLCdisplay owing to interaction with the interior surfaces of the display,a high (initial) resistance is very important in order to obtainacceptable service lives. In particular in the case of low-voltmixtures, it was hitherto impossible to achieve very high specificresistance values. It is furthermore important that the specificresistance exhibits the smallest possible increase with increasingtemperature and after heating and/or UV exposure. The low-temperatureproperties of the mixtures from the prior art are also particularlydisadvantageous. It is demanded that no crystallization and/or smecticphases occur, even at low temperatures, and the temperature dependenceof the viscosity is as low as possible. The MLC displays from the priorart thus do not meet today's requirements.

[0015] There thus continues to be a great demand for MLC displays havingvery high specific resistance at the same time as a largeworking-temperature range, short response times even at low temperaturesand low threshold voltage which do not have these disadvantages, or onlydo so to a reduced extent.

[0016] In TN (Schadt-Helfrich) cells, media are desired which facilitatethe following advantages in the cells:

[0017] extended nematic phase range (in particular down to lowtemperatures)

[0018] stable on storage, even at low temperatures

[0019] the ability to switch at extremely low temperatures (outdoor use,automobile, avionics)

[0020] increased resistance to UV radiation (longer service life).

[0021] The media available from the prior art do not allow theseadvantages to be achieved while simultaneously retaining the otherparameters.

[0022] In the case of supertwisted (STN) cells, media are desired whichenable greater multiplexability and/or lower threshold voltages and/orbroader nematic phase ranges (in particular at low temperatures). Tothis end, a further widening of the available parameter latitude(clearing point, smectic-nematic transition or melting point, viscosity,dielectric parameters, elastic parameters) is urgently desired.

[0023] The invention has an object of providing media, in particular forMLC, TN or STN displays of this type, which do not have theabove-mentioned disadvantages or only do so to a reduced extent, andpreferably simultaneously have very high specific resistance values andlow threshold voltages.

[0024] Upon further study of the specification and appended claims,further objects and advantages of this invention will become apparent tothose skilled in the art.

[0025] It has now been found that these and other objects can beachieved if media according to the invention are used in displays.

[0026] The invention thus relates to a liquid-crystalline medium basedon a mixture of polar compounds of positive dielectric anisotropy,characterized in that it comprises one or more compounds of the formulaI

[0027] in which

[0028] R¹ is a halogenated or unsubstituted alkyl or alkoxy radicalhaving from 1 to 15 carbon atoms, where, in addition, one or more CH₂groups in these radicals may each, independently of one another, bereplaced by —C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO— in such a way that Oatoms are not linked directly to one another,

[0029] X is F, Cl, CN, SF₅, a halogenated alkyl radical, a halogenatedalkenyl radical, a halogenated alkoxy radical or a halogenatedalkenyloxy radical having up to 6 carbon atoms,

[0030] L¹ and L² are each, independently of one another, H or F.

[0031] The compounds of the formula I have a broad range ofapplications. Depending on the choice of substituents, these compoundscan serve as base materials of which liquid-crystalline media arepredominantly composed; however, it is also possible to add compounds ofthe formula I liquid-crystalline base materials from other classes ofcompound in order, for example, to modify the dielectric and/or opticalanisotropy of a dielectric of this type and/or in order to optimize itsthreshold voltage and/or its viscosity.

[0032] In the pure state, the compounds of the formula I are colorlessand form liquid-crystalline mesophases in a temperature range which isfavorably located for electro-optical use. They are stable chemically,thermally and to light.

[0033] If R¹ in the formula I is an alkyl radical and/or an alkoxyradical, this may be straight-chain or branched. It is preferablystraight-chain, has 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly ispreferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy,butoxy, pentoxy, hexyloxy or heptyloxy, furthermore methyl, octyl,nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl,methoxy, octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy,tridecyloxy or tetradecyloxy.

[0034] Oxaalkyl is preferably straight-chain 2-oxapropyl(=methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-,3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or8-oxanonyl, or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

[0035] If R¹ is an alkyl radical in which one CH₂ group has beenreplaced by —CH═CH—, this may be straight-chain or branched. It ispreferably straight-chain and has 2 to 10 carbon atoms. Accordingly, itis in particular vinyl, prop-1- or prop-2-enyl, but-1-, -2- or -3-enyl,pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-,-2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or-7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-,-2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl.

[0036] If R¹ is an alkyl radical in which one CH₂ group has beenreplaced by —O— and one has been replaced by —CO—, these are preferablyadjacent. These thus contain an acyloxy group —CO—O— or an oxycarbonylgroup —O—CO. These are preferably straight-chain and have 2 to 6 carbonatoms. Accordingly, they are in particular acetoxy, propionyloxy,butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl,propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl,2-acetoxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

[0037] If R¹ is an alkyl radical in which one CH₂ group has beenreplaced by unsubstituted or substituted —CH═CH— and an adjacent CH₂group has been replaced by CO or CO—O or O—CO, this may bestraight-chain or branched. It is preferably straight-chain and has 4 to12 carbon atoms. Accordingly, it is in particular acryloyloxymethyl,2-acryloyloxyethyl, 3-acryloyloxypropyl, 4-acryloyloxybutyl,5-acryloyloxypentyl, 6-acryloyloxyhexyl, 7-acryloyloxyheptyl,8-acryloyloxyoctyl, 9-acryloyloxynonyl, 10-acryloyloxydecyl,methacryloyloxymethyl, 2-methacryloyloxyethyl, 3-methacryloyloxypropyl,4-methacryloyloxybutyl, 5-methacryloyloxypentyl, 6-methacryloyloxyhexyl,7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl or9-methacryloyloxynonyl.

[0038] If R¹ is an alkyl or alkenyl radical which is monosubstituted byCN or CF₃, this radical is preferably straight-chain. The substitutionby CN or CF₃ is in any desired position.

[0039] If R¹ is an alkyl or alkenyl radical which is at leastmonosubstituted by halogen, this radical is preferably straight-chain,and halogen is preferably F or Cl. In the case of polysubstitution,halogen is preferably F. The resultant radicals also includeperfluorinated radicals. In the case of monosubstitution, the fluorineor chlorine substituent may be in any desired position, but ispreferably in the co-position.

[0040] Compounds containing branched wing groups R¹ may occasionally beof importance owing to better solubility in the conventionalliquid-crystalline base materials, but in particular as chiral dopantsif they are optically active. Smectic compounds of this type aresuitable as components of ferroelectric materials.

[0041] Branched groups of this type generally contain not more than onechain branch. Preferred branched radicals R¹ are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexyloxy,1-methylhexyloxy and 1-methylheptyloxy.

[0042] If R¹ is an alkyl radical in which two or more CH₂ groups havebeen replaced by —O— and/or —CO—O—, this may be straight-chain orbranched. It is preferably branched and has from 3 to 12 carbon atoms.Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl,3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl,6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl,9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl,2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl,4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl,6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl,8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl,2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl,4,4-bis(ethoxycarbonyl)butyl or 5,5-bis(ethoxycarbonyl)hexyl.

[0043] The compounds of the formula I are prepared by methods known perse, as described in the literature (for example in the standard works,such as Houben-Weyl, Methoden der organischen Chemie [Methods of OrganicChemistry], Georg-Thieme-Verlag, Stuttgart), to be precise underreaction conditions which are known and suitable for the said reactions.Use can also be made here of variants which are known per se, but arenot mentioned here in greater detail. The compounds of the formula I canbe prepared, for example, as described in DE 4006921, WO 01/64667 and DE10105314.

[0044] The invention also relates to electro-optical displays (inparticular STN or MLC displays having two plane-parallel outer plates,which, together with a frame, form a cell, integrated non-linearelements for switching individual pixels on the outer plates, and anematic liquid-crystal mixture of positive dielectric anisotropy andhigh specific resistance which is located in the cell) which containmedia of this type, and to the use of these media for electro-opticalpurposes.

[0045] The liquid-crystal mixtures according to the invention enable asignificant widening of the available parameter latitude.

[0046] The achievable combinations of clearing point, viscosity at lowtemperature, thermal and UV stability and dielectric anisotropy are farsuperior to previous materials from the prior art.

[0047] The requirement for a high clearing point, a nematic phase at lowtemperature and a high Δε has hitherto only been achieved to aninadequate extent. Although mixtures such as, for example, ZLI-3119 havea comparable clearing point and comparably favorable viscosities, theyhave, however, a Δε of only +3. Other mixture systems have comparableviscosities and Δε values, but only have clearing points in the regionof 60° C.

[0048] The liquid-crystal mixtures according to the invention, forexample while retaining the nematic phase down to −20° C. and preferablydown to −30° C., particularly preferably down to −40° C., preferablyenable a clearing point above 60° C., more preferably above 65° C.,particularly preferably above 70° C., simultaneously dielectricanisotropy values Δε preferably of ≧6, more preferably ≧8, and a highvalue for the specific resistance to be achieved, enabling excellent STNand MLC displays to be obtained. In particular, the mixtures arecharacterized by low operating voltages. The TN thresholds arepreferably below 2.0 V, more preferably below 1.7 V, particularlypreferably <1.3 V.

[0049] It goes without saying that, through a suitable choice of thecomponents of the mixtures according to the invention, it is alsopossible for higher clearing points (for example above 110° C.) to beachieved at a higher threshold voltage or lower clearing points to beachieved at lower threshold voltages with retention of the otheradvantageous properties. At viscosities correspondingly increased onlyslightly, it is likewise possible to obtain mixtures having greater Δεand thus lower thresholds. The MLC displays according to the inventionpreferably operate at the first Gooch and Tarry transmission minimum [C.H. Gooch and H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch andH. A. Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975] are used, where,besides particularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German Patent 30 22 818), a lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistances to be achieved using the mixtures accordingto the invention at the first minimum than in the case of mixturescomprising cyano compounds. Through a suitable choice of the individualcomponents and their proportions by weight, the person skilled in theart is able to set the birefringence necessary for a pre-specified layerthickness of the MLC display using simple routine methods.

[0050] The flow viscosity ν₂₀ at 20° C. is preferably <60 mm² ·s⁻¹,particularly preferably <50 mm²·s⁻¹. The rotational viscosity γ₁ at 20°C. of the mixtures according to the invention is preferably <160 mPa·s,particularly preferably <150 mPa·s. The nematic phase range ispreferably at least 90°, in particular at least 100°. This rangepreferably extends at least from −20° to +80° C.

[0051] A short response time is desired in liquid-crystal displays. Thisapplies in particular to displays which are capable of videoreproduction. For displays of this type, response times (total:t_(on)+t_(off)) of at most 25 ms are required. The upper limit of theresponse time is determined by the image refresh frequency. Besides therotational viscosity γ₁, the tilt angle likewise affects the responsetime. In particular, mixtures comprising ≧20% of the compounds of theformula I exhibit a tilt angle of >2.5, preferably >3.0, compared withthe commercial product ZLI-4792 from Merck KGaA.

[0052] Measurements of the voltage holding ratio (HR) [S. Matsumoto etal., Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SIDConference, San Francisco, June 1984, p. 304 (1984); G. Weber et al.,Liquid Crystals 5, 1381 (1989)] have shown that mixtures according tothe invention comprising compounds of the formula I exhibit asignificantly smaller decrease in the HR with increasing temperaturethan, for example, analogous mixtures comprising cyanophenylcyclohexanesof the formula

[0053] or esters of the formula

[0054] instead of the compounds of the formula I.

[0055] The UV stability of the mixtures according to the invention isalso considerably better, i.e. they exhibit a significantly smallerdecrease in the HR on exposure to UV.

[0056] Particularly preferred compounds of the formula I are compoundsof the formulae I-1 to I-9:

[0057] in which R¹ is as defined in the formula I.

[0058] Of these preferred compounds, particular preference is given tothose of the formulae I-1, I-2, I-3 and I-4, in particular those of theformulae I-1 and 1-2.

[0059] Preferred embodiments are indicated below:

[0060] the medium comprises one, two or more compounds of the formulaeI-1 to 1-9;

[0061] the medium additionally comprises one or more compounds selectedfrom the group consisting of the general formulae IL to VI:

[0062] in which the individual radicals have the following meanings:

[0063] R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having upto 9 carbon atoms,

[0064] X⁰ is F, Cl, halogenated alkyl, halogenated alkenyl, halogenatedalkenyloxy or halogenated alkoxy having up to 6 carbon atoms,

[0065] Z⁰ is —C₂F₄—, —CF═CF—, —C₂H₄—, —(CH₂)₄—, —OCH₂—, —CH₂O—, —CF₂O—or —OCF₂—,

[0066] Y¹ to Y⁴ are each, independently of one another, H or F,

[0067] r is 0 or 1.

[0068] The compound of the formula IV is preferably

[0069] the medium additionally comprises one or more compounds selectedfrom the group consisting of the general formulae VII to XIII:

[0070] in which R⁰, X⁰ and Y¹⁻⁴ are each, independently of one another,as defined herein. X⁰ is preferably F, Cl, CF₃, OCF₃ or OCHF₂. R⁰ ispreferably alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 6carbon atoms.

[0071] The medium additionally comprises one or more compounds of theformulae E-a to E-d

[0072] in which R⁰ is as defined herein;

[0073] the proportion of the compounds of the formulae E-a to E-d ispreferably 10-30% by weight, in particular 15-25% by weight;

[0074] the proportion of compounds of the formulae I to VI together inthe mixture as a whole is at least 50% by weight;

[0075] the proportion of compounds of the formula I in the mixture as awhole is from 0.5 to 40% by weight, particularly preferably from 1 to30% by weight;

[0076] the proportion of compounds of the formulae II to VI in themixture as a whole is from 30 to 80% by weight;

[0077] the medium comprises compounds of the formulae II, III, IV, Vand/or VI;

[0078] R⁰ is straight-chain alkyl or alkenyl having from 2 to 7 carbonatoms;

[0079] the medium essentially consists of compounds of the formulae I toVI and XIII;

[0080] the medium comprises further compounds, preferably selected fromthe following group consisting of the general formulae XIV to XVIII:

[0081] in which R⁰ and X⁰ are as defined above. The 1,4-phenylene ringsmay additionally be substituted by CN, chlorine or fluorine. The1,4-phenylene rings are preferably monosubstituted or polysubstituted byfluorine atoms.

[0082] The medium additionally comprises one, two, three or more,preferably two or three, compounds of the formulae

[0083] in which “alkyl” and “alkyl*” are as defined below. Theproportion of the compounds of the formulae O1 and/or O2 in the mixturesaccording to the invention is preferably 5-10% by weight.

[0084] The medium preferably comprises 5-35% by weight of compound IVa.

[0085] The medium preferably comprises one, two or three compounds ofthe formula IVa in which X⁰ is F or OCF₃.

[0086] The medium preferably comprises one or more compounds of theformulae IIa to IIg

[0087] in which R⁰ is as defined above. In the compounds of the formulaeIIa-IIg, R⁰ is preferably methyl, ethyl, n-propyl, n-butyl or n-pentyl.

[0088] The medium preferably comprises one or more compounds of theformulae

[0089] in which R⁰ is as defined above.

[0090] The (I):II+III+IV+V+VI) ratio by weight is preferably from 1:10to 10:1.

[0091] The medium essentially consists of compounds selected from thegroup consisting of the general formulae I to XIII.

[0092] The proportion of the compounds of the formula IVb and/or IVc inwhich X⁰ is fluorine and R⁰ is CH₃, C₂H₅, n-C₃H₇, n-C₄H₉ or n-C₅H₁₁ inthe mixture as a whole is from 2 to 20% by weight, in particular from 2to 15% by weight.

[0093] The medium preferably comprises compounds of the formulae II toVI in which R⁰ is methyl.

[0094] The medium particularly preferably comprises compounds of theformulae

[0095] The medium preferably comprises one, two or more, preferably oneor two, dioxane compounds of the formulae

[0096] The medium additionally comprises one, two or more bicycliccompounds of the formulae Z1 to Z6

[0097] in which R^(1a) and R^(2a) are each, independently of oneanother, H, CH₃, C₂H₅ or n-C₃H₇. R⁰, alkyl and alkyl* are as defined inclaim 3 or as defined below.

[0098] Of the said bicyclic compounds, particular preference is given tothe compounds Z-1, Z-2, Z-5 and Z-6.

[0099] The medium additionally comprises one, two or more compoundshaving fused rings, of the formulae AN1to AN11:

[0100] in which R⁰ is as defined above.

[0101] It has been found that even a relatively small proportion ofcompounds of the formulae I mixed with conventional liquid-crystalmaterials, but in particular with one or more compounds of the formulaeII, Ii, IV, V and/or VI, results in a lowering of the threshold voltageand in low birefringence values, with broad nematic phases with lowsmectic-nematic transition temperatures being observed at the same time,improving the shelf life. Preference is given, in particular, tomixtures which, besides one or more compounds of the formulae I,comprise one or more compounds of the formula IV, in particularcompounds of the formula IVa in which X⁰ is F or OCF₃. The compounds ofthe formulae I to VI are colorless, stable and readily miscible with oneanother and with other liquid-crystalline materials.

[0102] The term “alkyl” or “alkyl*” covers straight-chain and branchedalkyl groups having 1-7 carbon atoms, in particular the straight-chaingroups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groupshaving 1-5 carbon atoms are generally preferred.

[0103] The term “alkenyl” covers straight-chain and branched alkenylgroups having 2-7 carbon atoms, in particular the straight-chain groups.Preferred alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl,C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇₋6-alkenyl, in particularC₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl. Examples ofparticularly preferred alkenyl groups are vinyl, 1E-propenyl,1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl,3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

[0104] The term “fluoroalkyl” preferably covers straight-chain groupshaving a terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl,3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and7-fluoroheptyl. However, other positions of the fluorine are notexcluded.

[0105] The term “oxaalkyl” preferably covers straight-chain radicals ofthe formula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. Preferably, n=1 and m is from1 to 6.

[0106] Through a suitable choice of the meanings of R⁰ and X⁰, theaddressing times, the threshold voltage, the steepness of thetransmission characteristic lines, etc., can be modified in the desiredmanner. For example, 1E-alkenyl radicals, 3E-alkenyl radicals,2E-alkenyloxy radicals and the like generally result in shorteraddressing times, improved nematic tendencies and a higher ratio of theelastic constants k₃₃ (bend) and k₁₁ (splay) compared with alkyl oralkoxy radicals. 4-alkenyl radicals, 3-alkenyl radicals and the likegenerally give lower threshold voltages and smaller values of k₃₃/k₁₁compared with alkyl and alkoxy radicals.

[0107] A —CH₂CH₂— group generally results in higher values of k₃₃/k₁₁compared with a single covalent bond. Higher values of k₃₃/k₁₁facilitate, for example, flatter transmission characteristic lines in TNcells with a 90° twist (in order to achieve grey shades) and steepertransmission characteristic lines in STN, SBE and OMI cells (greatermultiplexability), and vice versa.

[0108] The optimum mixing ratio of the compounds of the formulae I andII+m+IV+V+VI depends substantially on the desired properties, on thechoice of the components of the formulae I, II, III, IV, V and/or VI,and on the choice of any other components that may be present. Suitablemixing ratios within the range given above can easily be determined fromcase to case.

[0109] The total amount of compounds of the formulae I to XIII in themixtures according to the invention is not crucial. The mixtures cantherefore comprise one or more further components for the purposes ofoptimization of various properties. However, the observed effect on theaddressing times and the threshold voltage is generally greater, thehigher the total concentration of compounds of the formulae I to XIII.

[0110] In a particularly preferred embodiment, the media according tothe invention comprise compounds of the formulae II to VI (preferablyII, III and/or IV, in particular IVa) in which X⁰ is F, OCF₃, OCHF₂,OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favorable synergistic effect with thecompounds of the formulae I results in particularly advantageousproperties. In particular, mixtures comprising compounds of the formulaI and of the formula IVa are distinguished by their low thresholdvoltages.

[0111] The individual compounds of the formulae I to XVIII and theirsub-formulae which can be used in the media according to the inventionare either known or can be prepared analogously to the known compounds.

[0112] The construction of the MLC display according to the inventionfrom polarizers, electrode base plates and surface-treated electrodescorresponds to the conventional construction for displays of this type.The term conventional construction is broadly drawn here and also coversall derivatives and modifications of the MLC display, in particularincluding matrix display elements based on poly-Si TFT or MIM.

[0113] A significant difference between the displays according to theinvention and the conventional displays based on the twisted nematiccell consists, however, in the choice of the liquid-crystal parametersof the liquid-crystal layer.

[0114] The liquid-crystal mixtures which can be used in accordance withthe invention are prepared in a manner conventional per se. In general,the desired amount of the components used in the lesser amount isdissolved in the components making up the principal constituent,advantageously at elevated temperature. It is also possible to mixsolutions of the components in an organic solvent, for example inacetone, chloroform or methanol, and to remove the solvent again, forexample by distillation, after thorough mixing.

[0115] The dielectrics may also comprise further additives known to theperson skilled in the art and described in the literature. For example,0-15% of pleochroic dyes or chiral dopants can be added.

[0116] C denotes a crystalline phase, S a smectic phase, S_(C) a smecticC phase, N a nematic phase and I the isotropic phase.

[0117] V₁₀ denotes the voltage for 10% transmission (viewing angleperpendicular to the plate surface). t_(on) denotes the switch-on timeand t_(off) the switch-off time at an operating voltage corresponding to2.0 times the value of V₁₀. Δn denotes the optical anisotropy. Δεdenotes the dielectric anisotropy (Δε=ε∥−ε⊥, where ε∥ denotes thedielectric constant parallel to the longitudinal molecular axes and ε⊥denotes the dielectric constant perpendicular thereto). Theelectro-optical data are measured in a TN cell at the 1st minimum (i.e.at a d·Δn value of 0.5 μm) at 20° C., unless expressly stated otherwise.The optical data are measured at 20° C., unless expressly statedotherwise.

[0118] In the present application and in the examples below, thestructures of the liquid-crystal compounds are indicated by means ofacronyms, the transformation into chemical formulae taking place inaccordance with Tables A and B below. All radicals C_(n)H_(2n+1) andC_(m)H_(2m+1) are straight-chain alkyl radicals having n and m carbonatoms respectively; n and m are integers and are preferably 0, 1, 2, 3,4, 5, 6, 7, 8, 9, 10, 11 or 12. The coding in Table B is self-evident.In Table A, only the acronym for the parent structure is indicated. Inindividual cases, the acronym for the parent structure is followed,separated by a dash, by a code for the substituents R^(1*), R^(2*),L^(1*) and L^(2*): Code for R¹*, R²*, L¹*, L²* R¹* R²* L¹* L²* nmC_(n)H_(2n+1) C_(m)H_(2m+1) H H nOm OC_(n)H_(2n+1) C_(m)H_(2m+1) H HnO.m C_(n)H_(2n+1) OC_(m)H_(2m+1) H H n C_(n)H_(2n+1) CN H H nN.FC_(n)H_(2n+1) CN F H nN.F.F C_(n)H_(2n+1) CN F F nF C_(n)H_(2n+1) F H HnCl C_(n)H_(2n+1) Cl H H nOF OC_(n)H_(2n+1) F H H nF.F C_(n)H_(2n+1) F FH nF.F.F C_(n)H_(2n+1) F F F nmF C_(n)H_(2n+1) C_(m)H_(2m+1) F H nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₃.F C_(n)H_(2n+1) OCF₃ F H n-Vm C_(n)H_(2n+1)—CH═CH—C_(m)H_(2m+1) H H nV-Vm C_(n)H_(2n+1)— —CH═CH—C_(m)H_(2m+1) H HCH═CH—

[0119] Preferred mixture components are given in Tables A and B. TABLE A

PYP

PYRP

BCH

CBC

CCH

CCP

CPTP

CEPTP

ECCP

CECP

EPCH

PCH

PTP

BECH

EBCH

CPC

B

FET-nF

CGG

CGU

CFU

[0120] TABLE B

BCH-n.Fm

CFU-n-F

CBC-nmF

ECCP-nm

CCZU-n-F

T-nFm

CGU-n-F

CDU-n-F

DCU-n-F

CGG-n-F

CPZG-n-OT

CC-nV-Vm

CCP-Vn-m

CCG-V-F

CCP-nV-m

CC-n-V

CCQU-n-F

CC-n-V1

CCQG-n-F

CQCU-n-F

Dec-U-n-F

CWCU-n-F

CWCG-n-F

CCOC-n-m

CPTU-n-F

GPTU-n-F

PQU-n-F

PUQU-n-F

PGU-n-F

CGZP-n-OT

CCGU-n-F

CCQG-n-F

CUQU-n-F

CCCQU-n-F

[0121] Particular preference is given to liquid-crystalline mixtureswhich, besides the compounds of the formula I, comprise at least one,two, three or four compounds from Table B. TABLE C Table C showspossible dopants which are generally added to the mixtures according tothe invention.

C 15

CB 15

CM 21

R/S-811

CM 44

CM 45

CM 47

R/S-1011

R/S-3011

CN

R/S-2011

R/S-4011

[0122] TABLE D Stabilisers which can be added, for example, to themixtures according to the invention are mentioned below.

[0123] The entire disclosure of all applications, patents andpublications, cited herein and of corresponding German patentapplication No. 102 23 061.7, filed May 24, 2002 is incorporated byreference herein.

EXAMPLES

[0124] The following examples are intended to explain the inventionwithout restricting it. Above and below, percentages are percent byweight. All temperatures are given in degrees Celsius. m.p. denotesmelting point, cl.p. clearing point. Furthermore, C=crystalline state,N=nematic phase, S=smectic phase and I=isotropic phase. The data betweenthese symbols represent the transition temperatures. An denotes opticalanisotropy (589 nm, 20° C.). The flow viscosity ν₂₀ (mm²/sec) and therotational viscosity γ₁ (mPa·s) were each determined at 20° C. ExampleM1 CCH—5CF₃ 8.00% Clearing point [° C.]: 83.0 CCP—1F.F.F 6.00% Δn [589nm, 20° C.]: 0.0663 CCP—2F.F.F 8.00% Δε [1 kHz, 20° C.]: 11.7 CCP—3F.F.F8.00% γ₁ [mPa · s, 20° C.]: 157 CCOC-3-3 3.00% d · Δn [20° C.]: 0.50CCOC-3-5 3.00% Twist [°]: 90 CCOC-4-3 4.00% CCQU-1-F 8.00% CCQU-2-F9.00% CCQU-3-F 10.00%  CCCQU-3-F 9.00% CDU-2-F 8.00% CDU-3-F 8.00%Example M2 CCP—2F.F.F 8.00% Clearing point [° C.]: 83.0 CCH-34 12.00% Δn [589 nm, 20° C.]: 0.0659 DCU-3-F 6.00% Δε [1 kHz, 20° C.]: 12.3DCU-4-F 8.00% γ₁ [mPa · s, 20° C.]: 148 DCU-5-F 8.00% d · Δn [20° C.]:0.50 CECU-2-F 7.00% Twist [°]: 90 CECU-3-F 7.00% V₁₀ [V]: 1.16 CECU-5-F8.00% CCQU-1-F 8.00% CCQU-2-F 8.00% CCQU-3-F 10.00%  CCCQU-3-F 10.00% Example M3 CCP—2F.F.F 11.00%  Clearing point [° C.]: 85.5 CCP—3F.F.F11.00%  Δn [589 nm, 20° C.]: 0.0754 CCP—5F.F.F 6.00% Δε [1 kHz, 20° C.]:12.0 CCZU-2-F 4.00% γ₁ [mPa · s, 20° C.]: 155 CCZU-3-F 14.00%  d · Δn[20° C.]: 0.50 CCZU-5-F 4.00% Twist [°]: 90 CGU-2-F 7.00% V₁₀ [V]: 1.11CGU-3-F 4.00% CCH—5CF₃ 3.00% CCOC-4-3 3.00% CCQU-1-F 8.00% CCQU-2-F10.00%  CCQU-3-F 9.00% CCCQU-3-F 6.00% Example M4 CECU-2-F 8.00%Clearing point [° C.]: 82.0 CECU-3-F 8.00% Δn [589 nm, 20° C.]: 0.0749CECU-5-F 8.00% Δε [1 kHz, 20° C.]: 11.3 CCP—2F.F.F 9.00% γ₁ [mPa · s,20° C.]: 156 CCP—3F.F.F 10.00%  CCZU-2-F 3.00% CCZU-3-F 10.00%  CCZU-5-F3.00% CCQU-1-F 9.00% CCQU-2-F 6.00% CCQU-3-F 6.00% CCCQU-3-F 8.00%PCH—7F 5.00% BCH—3F.F.F 7.00% Example M5 CCP—2F.F 17.00%  Clearing point[° C.]: 83.0 CCP—3F.F 6.00% Δn [589 nm, 20° C.]: 0.0791 CCZU-2-F 3.00%γ₁ [mPa · s, 20° C.]: 100 CCZU-3-F 9.00% d · Δn [20° C.]: 0.50 CCP-319.00% Twist [°]: 90 CCH-34 12.00%  V₁₀ [V]: 1.65 PCH—7F.F.F 8.00% CCP—3F5.00% PCH-302 16.00%  PUQU-3-F 7.00% CCCQU-3-F 8.00% Example M6BCH—3F.F.F 18.00%  Clearing point [° C.]: 83.0 BCH—5F.F.F 10.00%  Δn[589 nm, 20° C.]: 0.1032 BCH—2F.F 9.00% γ₁ [mPa · s, 20° C.]: 158BCH—3F.F 9.00% d · Δn [20° C.]: 0.50 BCH—4F.F 5.00% Twist [°]: 90CCP—2F.F 8.00% V₁₀ [V]: 1.27 DCU-3-F 3.00% DCU-4-F 4.00% DCU-5-F 9.00%CCP-31 7.00% CCH-34 12.00%  CCCQU-3-F 6.00% Example M7 CCH-34 5.00%Clearing point [° C.]: 79.0 PCH—5Cl 8.00% Δn [589 nm, 20° C.]: 0.0923CCP—3F.F 13.00%  γ₁ [mPa · s, 20° C.]: 160 CCP—2F.F.F 9.00% d · Δn [20°C.]: 0.50 CCP—4F.F.F 6.00% Twist [°]: 90 BCH—2F.F 7.00% V₁₀ [V]: 1.22BCH—3F.F.F 15.00%  BCH—5F.F.F 7.00% DCU-3-F 3.00% DCU-4-F 5.00% DCU-5-F8.00% CCCQU-3-F 7.00% CCP-3 1 7.00% Example M8 CCH-34 6.00% Clearingpoint [° C.]: 80.0 PCH—5Cl 8.00% Δn [589 nm, 20° C.]: 0.0932 CCP—3F.F14.00%  γ₁ [mPa · s, 20° C.]: 135 CCP—4F.F 13.00%  d · Δn [20° C.]: 0.50CCP—2F.F.F 9.00% Twist [°]: 90 CCP—4F.F.F 8.50% V₁₀ [V]: 1.25 BCH—2F.F4.00% CCCQU-3-F 8.00% CCP-31 9.00% PUQU-2-F 6.00% PUQU-3-F 8.00%PUQU-5-F 6.50% Example M9 CCP—2F.F 4.50% Clearing point [° C.]: 78.5CCP—3F.F 13.00%  Δn [589 nm, 20° C.]: 0.0797 CCP—2F.F.F 6.50% γ₁ [mPa ·s, 20° C.]: 125 PCH—7F.F.F 9.00% d · Δn [20° C.]: 0.50 CCZU-2-F 3.00%Twist [°]: 90 CCZU-3-F 10.00%  V₁₀ [V]: 1.24 CCZU-4-F 3.00% DCU-3-F2.50% DCU-4-F 5.00% PUQU-2-F 6.00% PUQU-3-F 2.50% CCP-3F 3.00% CCP-319.00% CCH-34 5.00% PCH-302 10.00%  CCCQU-3-F 8.00% Example M10 CCP—3F.F12.50%  Clearing point [° C.]: 80.0 CCP—2F.F.F 5.00% Δn [589 nm, 20°C.]: 0.080 PCH—7F.F.F 8.00% γ₁ [mPa · s, 20° C.]: 140 CCZU-2-F 3.00% d ·Δn [20° C.]: 0.50 CCZU-3-F 10.00%  Twist [°]: 90 CCZU-4-F 3.00% V₁₀ [V]:1.25 DCU-3-F 3.00% DCU-4-F 5.00% DCU-5-F 8.00% BCH—3F.F.F 9.50% CCP-319.00% CCH-34 5.00% PCH-302 11.00%  CCCQU-3-F 8.00% Example M11 BCH—3F.F10.80%  Clearing point [° C.]: 107.5 BCH—5F.F 9.00% Δn [589 nm, 20° C.]:0.0970 ECCP—30CF₃ 4.50% ν [kHz, 20° C.]: 5.9 ECCP—50CF₃ 4.50% CBC—33F1.80% CBC—53F 1.80% CBC—55F 1.80% PCH—6F 7.20% PCH—7F 5.40% CCP—20CF₃7.20% CCP—30CF₃ 10.80%  CCP—40CF₃ 6.30% CCP—50CF₃ 9.90% PCH—5F 9.00%CCCQU-3-F 10.00%  Example M12 BCH—3F.F 10.91%  Clearing point [° C.]:106.0 BCH—5F.F 9.09% ECCP—30CF₃ 4.55% ECCP—50CF₃ 4.55% CBC—33F 1.82%CBC—53F 1.82% CBC—55F 1.82% PCH—6F 7.27% PCH—7F 5.45% CCP—20CF₃ 7.27%CCP—30CF₃ 10.91%  CCP—40CF₃ 6.36% CCP—50CF₃ 10.00%  PCH—5F 9.09%CCCQU-3-F 9.10% Example M13 CCH-301 12.59%  Clearing point [° C.]: 97.5CCH—3CF₃ 7.20% Δn [589 nm, 20° C.]: 0.0639 CCH-501 9.89% ν [kHz, 20°C.]: 7.3 CCP—2F.F.F 8.99% CCP—3F.F.F 11.69%  CCP—5F.F.F 4.50% CCPC-332.70% CCZU-2-F 4.50% CCZU-3-F 15.29%  CCZU-5-F 4.50% CH-33 2.70% CH-352.70% CH-43 2.70% CCCQU-3-F 10.06% 

[0125] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

[0126] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A liquid-crystalline medium comprising a mixture of polar compoundsof positive dielectric anisotropy, wherein the medium comprises one ormore compounds of the formula I

in which R¹ is a halogenated or unsubstituted alkyl or alkoxy radicalhaving from 1 to 15 carbon atoms, where one or more CH₂ groups in theseradicals are optionally, independently of one another, replaced by—C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO— in such a way that O atoms are notlinked directly to one another, X is F, Cl, CN, a halogenated alkylradical, a halogenated alkenyl radical, a halogenated alkoxy radical ora halogenated alkenyloxy radical having 1 to 6 carbon atoms, L¹ and L²are each, independently of one another, H or F.
 2. A liquid-crystallinemedium according to claim 1, which comprises one, two or more compoundsof the formulae I-1 to I-9:

in which R¹ is as defined in claim
 1. 3. A liquid-crystalline mediumaccording to claim 1, which further comprises one or more compoundsselected from the group consisting of those of the formulae II, III, IV,V and VI:

in which the individual radicals have the following meanings: R⁰ isn-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1 to 9 carbonatoms, X⁰ is F, Cl, halogenated alkyl, halogenated alkenyl, halogenatedalkenyloxy or halogenated alkoxy having 1 to 6 carbon atoms, Z⁰ is—C₂F₄—, —CF═CF—, —C₂H₄—, —(CH₂)₄—, —OCH₂—, —CH₂O—, —CF₂O— or —OCF₂—, Y¹to Y⁴ are each, independently of one another, H or F, and r is 0 or 1.4. A liquid medium according to claim 2 which further comprises one ormore compounds selected from the group consisting of those of theformulae II, III, IV, V and VI:

in which the individual radicals have the following meanings: R⁰ isn-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1 to 9 carbonatoms, X⁰ is F, Cl, halogenated alkyl, halogenated alkenyl, halogenatedalkenyloxy or halogenated alkoxy having 1 to 6 carbon atoms, Z⁰ is—C₂F₄—, —CF═CF—, —C₂H₄—, —(CH₂)₄—, —OCH₂—, —CH₂O—, —CF₂O— or —OCF₂—, Y¹to Y⁴ are each, independently of one another, H or F, and r is 0 or 1.5. A liquid-crystalline medium according to claim 3, wherein theproportion of compounds of the formulae I to VI together in the mixtureas a whole is at least 50% by weight.
 6. A liquid-crystalline mediumaccording to claim 1, which further comprises one or more compounds ofthe formulae E-a to E-d:

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1to 9 carbon atoms.
 7. A liquid-crystalline medium according to claim 1,which further comprises one or more compounds of the formulae IIa to IIg

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1to 9 carbon atoms.
 8. A liquid-crystalline medium according to claim 1,which further comprises one or more compounds of the formulae O1 and O2:

in which alkyl and alkyl* are each, independently of one another, astraight-chain or branched alkyl group having 1-7 carbon atoms.
 9. Aliquid-crystalline medium according to claim 1, which further comprisesone or more dioxane compounds of the formulae D1 and/or D2:

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1to 9 carbon atoms.
 10. A liquid-crystalline medium according to claim 1,wherein the proportion of compounds of the formula I in the mixture as awhole is from 0.5 to 40% by weight.
 11. An electro-opticalliquid-crystal display containing a liquid-crystalline medium accordingto claim
 1. 12. A liquid-crystalline medium according to claim 2,wherein the medium comprises one or more compounds of the formulae I-1,I-2, I-3 or I-4.
 13. A liquid-crystalline medium according to claim 2,wherein the medium comprises one or more compounds of the formulae I-1or I-2.
 14. A liquid-crystalline medium according to claim 6, whereinthe proportion of the compounds of the formulae E-a to E-d in the mediumis 10-30% by weight.
 15. A liquid-crystalline medium according to claim8, wherein the proportion of the compounds of the formulae O1 and/or O2in the mixtures according to the invention is 5-10% by weight.
 16. Aliquid-crystalline medium according to claim 1, wherein the mediumcomprises 5-35% by weight of a compound of the formula IVa:

in which the individual radicals have the following meanings: R⁰ isn-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1 to 9 carbonatoms, X⁰ is F, Cl, halogenated alkyl, halogenated alkenyl, halogenatedalkenyloxy or halogenated alkoxy having 1 to 6 carbon atoms.
 17. Themedium of claim 16, which comprises one, two or three compounds of theformula IVa in which X⁰ is F or OCF₃.